Update contrib/mkimage/debootstrap to remove /var/lib/apt/lists (trimming at least 40MB and forcing "apt-get update" in dependent images before packages can be installed)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6690 from tianon/remove-apt-lists

Update contrib/mkimage/debootstrap to remove /var/lib/apt/lists